Help Balancing a Homebrew final boss

[Danielqueue1]

ANYONE IN THE AEGIS FALL CAMPAIGN STOP READING.

So I have been running a homebrew campaign over the last few years that is coming to a final conclusion. There is a reluctant villain that has been the terror of the party since level 1.
they were all knocked out by him at the end of first session. they heard the stories of his army, defended a keep against him until [[plot device]] showed up. fought him in a "survive for 5 combat rounds until the magic teleports you to safety." fight. (there was lots of dodging) had a pleasant, but stressful conversation with him in his own fortress (Strahd style) narrowly escaped his Elite eliminator squads. had a PC turn against them (willingly) to work for him. snuck in the back entrance to a fortress to take an artifact of power before him and his army broke through and claimed it for themselves. chopped off one of the Tiefling's horns so that when he got back to the party, his wizards could scry on them.
long story short, I cannot afford to have the final confrontation anything less than memorable.

I was planning on using the DMG directions for CR but there are a few things about this character that don't really fit the mold.

the Concept; Ultimate swordsman, has a contingency for everything, but a distrust of most magic he doesn't control so he only has a few trusted magic users at his side if he goes out to battle. He bears an ancient sword that was forged before the material plane was formed by ((plot))

the battle;

A balanced party of 6-7 level 17 PCs
•Life domain Cleric
•Thief Rogue with a splash of sorcerer
•Ranger Rogue
•Moon Druid
•Long death monk
•Paladin/warlock (Palock?)
•PC currently being built (last one got killed by a lich.)
the party has 4-5 characters who can heal and they have shown solid defensive tactics throughout, so it is good to up the threat a bit.

Vs
this villain
+ his two Bruiser body guards, They give the boss advantage when within 5 feet of a target, and regenerate, so if the party wants to be rid of them they will have to focus them down.
+ a wizard with greater invisibilty (self cast) and good counter spell. will generally be an annoyance and make sure the boss doesn't get shut down.
+? maybe one other Ally they haven't met.
all of this in the area of a forbiddance zone to prevent summons and planar allies. PCs will be at full strength.

The build; his defenses I am happy with, I just need help with balancing the offences.

he has two forms of offence, "forms" and "sword strikes" he cannot use the same attack more than once in a round

SWORD STRIKES
Strike: heavy hitting single target attack.
Cleave: hits two targets
Break: on a hit permanently reduces target's AC by 1d4-1 (possibly zero)
Crush: on a hit target makes CON save vs stun.
Maim: on a hit target makes CON save vs 1 level of exhaustion
Purge: deals low damage, but destroys creations of magical force and counts as a Dispel Magic on it's target.
END: low damage, but deals HEAVY damage against incapacitated foes

naturally if some of these were spammed they would be OP, but he cannot use any of them more than once per round, so he wont be hitting someone with three levels of exhaustion in a single round. the party already scried on him using these attacks so they know what they do, so I can't change them, but they do not know the damage dealt so I can change that freely.

FORMS

Folding the Air: ranged attack. hits very hard, STR Save vs knock back and prone.
Moon Rising over Water: area line attack 25 feet x 5 feet. hits everything in the line and turns the area into difficult terrain
Whirlwind on the Mountain: area attack All targets within 10 feet STR save vs Knockback and prone

Actions
Multi attack: (name) uses 3 Sword strikes
Sword form: (name) uses one "Form"

legendary actions (3)
moves up to 5 feet without provoking an attack of opportunity and uses one Sword Strike (costs 1)
(defensive option) (costs 1)
uses a "form" (costs 2)

legendary saves (3)

KISSING THE ADDER
I haven't figured out exactly how to balance this, the Idea is a ranged counter attack similar to the Monk's deflect missiles, but I want it to have it's own flavor and not be crushed by the action economy. all but two (or three) of the party have ranged attacks, so I don't want him to be helpless against kiting and their superior action economy, so here's the idea

on a ranged attack miss he can immediately target the attacker with "Kissing the Adder" dealing X damage if it hits

option 1; uses reaction, but hits like a truck.
option 2; consumes a legendary action, hits hard
option 2; has a specific number of uses per round, hits pretty well.
option 3; can be used once per turn, needling damage.


I know this is a complicated set up, but this Is my version of the "DM's Superbeast." and the final standard combat in the campaign, so they won't have to worry about any of the after effects like exhaustion and AC penalties. I want them to feel accomplished after beating him and I am a big proponent on the idea that heroes are "measured by their villains."

TL;DR how much damage should each of those attacks do? and what do you recommend for setting up "kissing the Adder?"

[Man_Over_Game]

Big into Wheel of Time? Props to that.

My recommendation is to focus on things that are temporary setbacks that can be resolved from player interaction. Exhaustion, unfortunately, doesn't fall into this mix. While it's a great mechanic for a campaign to utilize frequently, it's nearly impossible to get rid of in combat.

But Poison is available and does the same thing. Or Prone, Fear, Restrained. Many effects from enemies require a good saving throw roll from the victim, or an action from one of his friends to rescue him. Restraining someone by a grasp of Air is definitely something Rand would do, and if you make it tangible, you could allow someone to attack it to free their friend.

As for Kiss of the Adder, why not have two options:

1. Disadvantage on the attack, followed by a teleport as a reaction. If teleport is too magical, then just give him high movement with disengage action.
2. Resist that attack, and all damage that share the same types as the initial attack until the start of his next turn
3. The attack does full damage, but his initiative is exactly RIGHT NOW (Legendary action).

The teleport gives him mobility to avoid bad situations, but doesn't give him access to his shielding ability. Getting out of Line of Sight is one way to nullify their action economy
The shielding ability is good on multiple damage type attacks, and forces the players to change up their fighting methods.

[Danielqueue1]

I did like exhaustion specifically for the fact that it is difficult to remove. Three of the party are immune to poisons, 2 from class 1 from item, so it would be useless against half the group. The cleric has greater restoration and gems to use it so if it stacks too high it won't be the death of them. And there aren't going to be any combats after this so taking a week off to recover will be easy.

I like the kissing the adder ideas (and yes I took some inspiration from wheel of time) but what about the damage?

[Man_Over_Game]

Unfortunately, damage is hard for me to balance at that level, especially since the additional mobs aren't mentioned much, and I'm not sure how tanky you want this guy to be.

I can say from experience that if there are no surprises then the fight doesn't have tension, but a surprise at the wrong time means the party dies in a one-sided battle.

My recommendation is to have frequent light damage attacks, to allow the players the chance to react, while allowing the boss to have high health. They have limited resources, where monsters generally don't, so a high accuracy, high health, low damage boss makes things interesting. To add additional scares and tension, implement a feature on all of his attacks that they deal extra damage to targets with over 50% or max health (your call as to which). That initial shock, as their paladin hits half health from a single blow at the start of combat, will force them to play defensive and cautious right at the get-go. They don't need to know that the damage is less lethal the closer they are to dying.